This op-ed is co-authored by Jason Stearns, director of the New York University-based Congo Research Group and a former coordinator of the United Nations Group of Experts on Congo.

In mid-March, armed men walked Michael J. Sharp and Zaida Catalán through the savanna in the Democratic Republic of Congo’s Kasai region, sat them down and shot them. A low-resolution video captured the executions. Mr. Sharp, an American, and Ms. Catalán, a Swede, were United Nations sanctions monitors, charged with finding out who was responsible for human rights abuses and supporting armed groups in the Democratic Republic of Congo.

António Guterres, the United Nations’ secretary general, promised that his organization would “do everything possible to ensure that justice is done.” But more than two months have passed, and his words are beginning to ring hollow. Neither the United Nations nor the Swedish and American governments have done enough to get to the bottom of who killed Ms. Catalán and Mr. Sharp, who gave the orders and why. The four Congolese who had accompanied them — their interpreter Betu Tshintela; a motorbike driver, Isaac Kabuayi; and two unidentified motorbike drivers — are still missing.

The killings of the United Nations investigators were exceptional on many levels. First, the personal: Both were young, remarkable individuals. Mr. Sharp was only 34, but he was the coordinator of the highly regarded group and had spent three years trekking through eastern Congo, persuading rebels to put down their weapons. Ms. Catalán, 36, was a passionate human rights and environmental activist who had been a youth leader of the Green Party in Sweden and spent years working for human rights and security reform in Afghanistan, Palestine and eastern Congo.

Then there was the historical significance. They were the first United Nations investigators monitoring sanctions to have been killed in the line of duty since the United Nations imposed the first sanctions on Southern Rhodesia (now Zimbabwe) in 1966. Since then, hundreds of sanctions monitors have been deployed to report on everything from the Afghan Taliban to Iraq’s nuclear program to rebels in Sierra Leone.

Finally, and most tragically, their deaths are a reminder of how little attention is paid to the killings of hundreds of Congolese in the Kasai region since last August, which is what Mr. Sharp and Ms. Catalán were investigating. Human rights teams have uncovered at least 42 mass graves in the region, a majority of which are believed to be the work of the Congolese military. In the past 10 months, some 1.3 million people have been displaced by violence there — more than anywhere else in the world during the same period. More than 600 schools have been attacked or destroyed, and more than 1.5 million children are affected by the violence. But almost nothing has been done to provide justice for the victims.

The longer the United Nations waits, the harder any investigation will be as key evidence or witnesses could disappear. Reports have already emerged of soldiers digging up the mass graves to cover up the traces of their crimes.

At a meeting of the United Nations Human Rights Council in March, the high commissioner for human rights called for an inquiry into the situation in the Kasai region. European countries agreed that such an inquiry was necessary, but efforts were stymied by Congo’s government and several African countries that said the Congolese justice system should be given a chance to conduct its own investigation into the violence.

More than three months later, the Congolese government has failed to produce a credible investigation, and the United Nations human rights office in Congo has not had the access or cooperation needed to provide meaningful support. The high commissioner reiterated his call for an international investigation last week, and council members have another opportunity to establish an investigation during their current session.

As for Mr. Sharp and Ms. Catalán, the secretary general has appointed a board of inquiry, but it is primarily focused on whether United Nations security protocols were adequate and followed. It has neither the mandate nor the capacity to investigate who was responsible for the killings. The United States and Sweden have begun their own investigations, but they would most likely need to depend on Congolese government collaboration to interview witnesses, obtain phone records and visit the crime scene.

More needs to be done.

President Joseph Kabila, who was supposed to step down last year at the end of his second term, has shown little will to bring those responsible for the massacres of Congolese or the murders of the United Nations investigators to justice. Members of the security forces have been directly involved in the violence, and the Congolese government has a long record of meddling in sensitive judicial cases.

The impetus for justice will have to come from outside. Special United Nations investigations into the murders of Mr. Sharp and Ms. Catalán, as well as the broader violence in the Kasais, could engage in field investigations, gather evidence and identify suspects. Public reports with findings would be a basis to press for all those responsible for the killings and other abuses, regardless of position or rank, to face justice. Criminal investigations in the United States and Sweden or by the International Criminal Court could ultimately lead to arrests and prosecutions.

It is important to remember that on Congo, there are no excuses for international inaction. This is not Syria, where Russian support for the government and the threat of the Islamic State have created a geopolitical stalemate. Congo has few committed and powerful allies. In fact, a large part of its budget is supported by the very Western governments demanding accountability, and its army is backed up in the east by the largest United Nations peacekeeping force in the world. These paradoxes point to a critical, uncomfortable truth: In Congo, the biggest stumbling blocks can be apathy and a lack of political will. We can find out who killed Mr. Sharp and Ms. Catalán just as we can deliver justice for the hundreds of Congolese who have lost their lives in the Kasais. We just have to care enough.

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